A major obstacle for various approaches in regenerative medicine aimed at replacing lost organ functions is the shortage of available cell sources. The availability of such approaches is often limited by the shortage of donor tissue. Stem cells, including embryonic or bone marrow derived mesenchymal stem cells, are a potential cell source for regenerative medicine (1-4). Human embryonic stem (hES) cells are capable of indefinite self-renewal and are pluripotent; that is, these cells are able to differentiate into practically every type of cell found in the organism from which they are derived (17, 18). Human mesenchymal stem (hMS) cells are multi-potent and can differentiate into various cell lineages of mesenchymal tissues including, bone, cartilage, fat, tendon, muscle, adipocytes, chondrocytes, and osteocytes (19, 20). hES and hMS cells accordingly have tremendous potential to provide different cell types for use in a variety of medical and research purposes.
Techniques for in vitro differentiation of stem cells into particular cell lineages using specific proteins or chemical molecules have been well-studied. However, achieving a large population of differentiated functional cells remains a challenge.
To date, differentiation of stem cells in vitro has generally been limited to reliance on non-directed cell differentiation or inefficient methods for inducing differentiation that require the addition of various growth factors and supplements to the cell culture and that tend to produce a low percentage of desired differentiated cells (21, 40).
Similarly, achieving in vitro undifferentiated proliferation of hES cells suitable for clinical applications has been challenging. Traditional culture methods for proliferating human stem cells require the use of mouse embryonic fibroblasts (MEFs) as a feeder layer, which could result in cross-contamination of the human stem cell population with animal components (22). Contamination of stem cells or their differentiated derivatives with animal components increases the likelihood of immune rejection during regenerative therapies (23, 24).